According to a conventional combustion apparatus such as a conventional water heater or a conventional heat source device for a room heater, when airflow resistance in a circulation path of combustion exhaust gas ejected from a burner is too low, pressure around a combustion area of the burner largely fluctuates in an intensive combustion mode of the burner. As a result, combustion balance is disturbed, vibration combustion occurs, and noise increases. Accordingly, it has been proposed to provide, on a downstream side end of a casing that forms the circulation path of the combustion exhaust gas, an exhaust resistance plate having a plurality of small circular ventilation ports, so as to regulate an exhaust flow rate of the combustion exhaust gas discharged from the casing. (For example, Japanese Unexamined Patent Publication No. H04-113167 A)
On the other hand, there has been also known a so-called downward combustion type combustion apparatus configured to flow combustion exhaust gas through a heat exchanger from top to bottom. In this kind of the combustion apparatus, when the exhaust resistance plate is provided beneath the heat exchanger, i.e., on a downstream side in the casing, drain, which is generated on a surface of the heat exchanger during a combustion operation, drips onto the exhaust resistance plate and forms a film of water on the ventilation ports, whereby the ventilation ports may be closed.
In particular, as the above-described conventional combustion apparatus uses the exhaust resistance plate having the small circular ventilation ports, the drain is likely to be retained at circumferential edges of the circular ventilation ports due to surface tension. Therefore, even a minute amount of drain may close the ventilation ports. When the ventilation ports are closed by the film of water, the exhaust flow rate from the casing reduces, and accordingly input to the burner (a supply amount of the fuel gas on a heat value basis) reduces. As a result, the heat exchanger hardly recovers an necessary amount of heat, and a heat medium at a desired temperature may not be supplied to a hot-water supplying terminal.